1. Field of the Invention
This invention relates to a magnetic read/write system and, more particularly, to a magnetic recording-reproducing device capable of adjusting electrically the phase of a pulse indicating the direction/position of rotation of a head.
2. Description of the Prior Art
In the video tape recorder (VTR), two recording/reproducing heads (video heads) are mounted with a spacing of 180.degree. left therebetween on a rotary drum, a magnetic tape is run while being caused to contact obliquely with the cylinder surface of the rotary drum so that the running track (named "video track") of each head on the magnetic tape is inclined with respect to the lengthwise direction of the tape and the running tracks of respective heads are offset from each other successively by one track pitch, and the two heads are switched and put into operation alternately, whereby a signal is recorded on the magnetic tape and/or read therefrom to reproduce the same. That is, the signal is successively recorded on the magnetic tape and/or read therefrom while the two heads are scanning alternately, in such a mode of operation as to cut a thread, the surface of the magnetic tape which runs while being coiled round the cylinder surface of the rotary drum. Because the head is rotated at a rotating speed (30 rps) identical to the frame frequency of the TV signal, the amount of information of the TV signal corresponding to one field is recorded in one video track during one rotation.
As is well known in the art, it is required that a recording signal be applied to alternate head and recorded on the magnetic tape field by field or a signal be pulled out from alternate head to reproduce the same, and recording or reading of the one-field information carried out by alternate head be commenced at a definite position at which the magnetic tape starts to contact with the rotary drum. Namely, a signal indicating that the head has come to the definite position must be generated always, on the basis of which several processes, such as switching of the heads and branching of the signal, must be commenced.
In view of the foregoing circumstances, according to the prior art, magnet segments of N-pole and S-pole are attached at a spacing of 180.degree. to a rotary member rotatable integrally together with the rotary drum, and a magnetic sensor (such as, pickup coil or Hall element) is provided on a fixing member with leaving a certain gap between it and the rotating path of the magnet segments; these making up a pulse generating unit. By the foregoing configuration, positive-polarity and negative-polarity pulses (referred to as a head position pulse) are generated alternately by the magnetic sensor in response to rotation of the rotary drum each time the N-pole and S-pole magnet segments come to opposition to the magnetic sensor, and in this connection, the mounting positions of the magnet segments and/or magnetic sensor are adjusted so that the head position pulse may be generated each time the head comes to the definite position.
However, the work of adjusting the phase (timing) of generation of the head position pulse through mechanical adjustment of the mounting positions of the magnet segments and/or magnetic sensor is very troublesome and needs a comparatively long time for adjustment.